This invention relates generally to the production of corn, and particularly to the production of inbred and hybrid tetraploid corn.
In order to understand and appreciate the significance of the claimed invention and the techniques described herein, it is necessary to understand certain basic principles of genetics and plant breeding.
The genetic information of higher plants is contained in chromosomes located inside the nucleus of individual plant cells. This is called the "genome" of the plant. The genome of plants usually exists within each cell as sets of similar or homologous chromosomes. The chromosomes contain the basic units of heredity, the gene. Genes occupy fixed positions on the chromosomes, and each gene has a specific influence on the expression of a particular characteristic or characteristics of the plant. Alternative forms of the same genes are known as alleles, and different alleles of a gene may be found on homologous chromosomes. If homologous chromosomes contain the same alleles for a particular gene, the plant is homozygous for that gene and the characteristic that it controls. If homologous chromosomes contain different alleles for the same gene, the plant is heterozygous for that gene and the characteristic that it controls.
The number of chromosome sets in the cell of a plant is called the plant's "ploidy" and is often designated a multiple of "N" where "N" denotes the number of chromosomes per set. A plant with N chromosomes in its cell nucleus is termed "haploid," a plant with 2N chromosomes is termed "diploid," and a plant with 4N chromosomes is termed "tetraploid," and so on. The term "polyploid" refers to multiple chromosome sets in excess of two. The numerical value of N differs from species to species. In corn, N is normally equal to 10. Hence, diploid corn has 2N or 20 chromosomes, tetraploid corn has 4N or 40 chromosomes, and polyploid corn has 3 or more sets of N chromosomes.
Gametes or reproductive cells have nuclei that ordinarily contain half of the total sets of chromosomes and, therefore, one half of the number of chromosomes. Thus, if a plant is diploid, its gametes will be haploid; if it is tetraploid, its gametes will be diploid. The fusion of gametes from a male parent and a female parent produces a zygote, which develops into a seed.
The corn that conventionally has been grown for human or animal food, or as a source of raw materials or chemicals, is diploid (2N=20). Production of biomass by diploid corn, in the form of grain (the ears) and stover (the remainder of the plant), has conventionally been increased by the use of fertilizers, pesticides, and selective breeding. Although the application of fertilizers and the elimination of insect pests have resulted in increased yields of biomass, the effectiveness of all of these methods is limited by the genetic makeup of the plant.
For this reason, much research in improving corn production has concentrated on selective breeding. Selective breeding is conducted by selecting plants that have desirable traits, such as resistance to disease, increased production of biomass, or high fertility, and inbreeding those plants or outcrossing them with other plants having desirable characteristics.
Inbreeding is a technique that produces maximum genetic uniformity in a genetically variable, cross-pollinated species, such as corn. Inbred lines are derived by a process of self-pollination and selection, usually over 5 or more generations, so that allelic pairs of genes on homologous chromosome pairs are homozygous or identical. The degree of inbreeding (homozygosity) in a line is approached at the rate of about 50% per generation so that by the second generation, plants are about 75% homozygous and by the sixth generation about 98% homozygous. Thereafter, all plants derived from self-pollination, sibling pollination, or random crossing with others in the inbred line theoretically should be essentially genetically identical and, therefore, should be essentially homozygous and uniform in appearance. However, the determination of whether a uniform, stable, and essentially homozygous line exists requires the putative inbred line to be field tested and examined for variation in its significant characteristics. If the degree of variation is unacceptable, i.e., too high, the process of self-pollination and selection must be continued for additional generations until the degree of variation in the line's significant characteristics is accept- able.
Although inbreeding results in genetic uniformity, it also results in a reduction in performance, yield, and plant size for species that normally reproduce by cross-pollination. This reduction is known as inbreeding depression and is the reason that the uniform inbred lines are not grown as a commercial crop.
Outcrossing or hybridization involves cross-pollination of plants differing in genetic constitution. Thus, a hybrid is the progeny of genetically unlike parents. Hybridization results in the progeny expressing a variety of characteristics, and the plant breeder will select plants expressing desirable characteristics for further breeding. Some examples of desirable characteristics sought through hybridization are increased yield, resistance to diseases and insects, faster maturity, and specific changes in the size, shape, and height of the plant. Random hybridization, however, produces a commercially undesirable level of variation in the genetic makeup and characteristics of the resulting plants.
The principal technique used for the production of uniform plant populations that do not show inbreeding depression is the hybridization of two inbred lines to produce hybrid seeds which, upon planting and growth, produce uniform first generation (F.sub.1) hybrid plants. Because of hybrid vigor, maximum yield as well as uniformity is achieved.
The development of the desired hybrid seeds is a specialized and highly skilled procedure. Crosses of many different inbred lines must be done, the hybrid seeds must be grown, and the plants must be evaluated for desired characteristics. Often, a program to develop a new hybrid seed starts with the development of new inbred lines, which are then crossed to determine which ones will produce the desired hybrid. A great many different inbred lines may be evaluated. The total time to develop a new hybrid seed can be as much as 7-10 years.
Once the inbred lines are developed and the best ones are selected by evaluating the hybrids they produce, new hybrid seeds are obtained each generation by crossing the originally selected inbred parents. The desired hybrid cannot be reproduced from self-pollination, or by crossing with another F.sub.1 hybrid because the recombination of genes will produce progeny that are extremely variable in maturity, quality, and yield. As a result, farmers usually purchase the hybrid seeds used to produce F.sub.1 hybrid plants from a commercial seed company.
In diploid corn, improvements by selective breeding have been slow, since only one to three generations of corn may be propagated each year. Therefore, relatively minor improvements in diploid corn, such as increases in biomass, have been obtained only after years of rigorous work.
Although tetraploid corn has been known for approximately 50 years, it has been cultivated for the purposes of curiosity and cytological and genetic studies rather than for the production of grain or large quantities of biomass. Thus, there is limited discussion in the scientific literature of the grain or biomass production characteristics of tetraploid corn, and statements about these characteristics have been made without supporting data. One report concludes that tetraploid corn is inferior to diploid corn on the basis of grain yield. Such statements provide little incentive for developing inbred lines of tetraploid corn for the purpose of producing hybrid tetraploid corn seed for commercial uses. The inventor is unaware of the development of any inbred lines (as defined herein) of tetraploid corn prior to the present invention.
Tetraploid corn was studied as early as the 1930's. In 1932, L.F. Randolf reported on the use of high temperature to induce tetraploidy in corn. Randolph, L.F., Proc. N.A.S. 18:222-229, 1932. In 1935, Randolph stated that tetraploids ". . . are of about the same height and have a similar habit of growth [as diploid corn]. However, their stalks are thicker and sturdier and the leaves are somewhat broader and thicker . . . the ears and kernels of tetraploid maize are distinctly larger than those of comparable diploid stocks." Randolph L. F., J. of Agricultural Research 50(7):591-605, 1935. However, Randolph presented no quantitative data to substantiate these statements. In 1944,Randolph published data that supported his statement with respect to leaf characteristics only. Randolph, L. F., J. of Agricultural Research 69:47-76, 1944.
Characteristics of several types of tetraploid corn were examined in the 1950's and 1960's by Dudley and Alexander at the University of Illinois. Their findings as to grain yield, seed set, plant height, ear height, weight per 100 kernels, number of kernel rows, and ear length of tetraploid corn were reported in Dudley, J. W. and D. E. Alexander, Crop Science 9:613-615, 1969. This study concluded that the highest grain yield obtained from the tetraploids was not competitive with good diploid hybrids. No diploids were included in the experiment for comparison.
In 1974, Rice and Dudley reported on inbreeding depression in tetraploid corn resulting from selfing and from full-sib mating. Rice, J. S. and J. W. Dudley, Crop Science 14:390-393, 1974. However, they reported their data on a population basis rather than for individual lines. No data on the variation of important characteristics were reported for individual lines, which would have been necessary to demonstrate the creation of inbred lines.
Thus, although tetraploid corn has been documented for over 50 years, the inventor is unaware of any published reports supported by data of either inbred lines (as defined herein) of tetraploid corn or hybrid tetraploid corn produced by crossing inbred lines, nor is the inventor aware of any published data documenting that tetraploid corn is superior to diploid corn for grain, stover, and whole plant production. In fact, as indicated above, the published work of some breeders indicates that yields from tetraploid corn are inferior to that of good diploid hybrids.
From the comparison of certain populations of tetraploid corn and their first generation intercross progeny with certain lines of hybrid diploid corn, it has been discovered that tetraploid corn plants show a biomass yield superior to diploid plants. Such increase in yield is on the order of 5-20%. This unexpected and surprising discovery provided the motivation to develop inbred lines of tetraploid corn that may be crossed to produce hybrid seeds and plants. Because of the greater biomass yields, hybrid tetraploid corn plants will be attractive as a source of food for livestock, as silage, and as raw material for the production of biomass-derived chemicals, such as ethanol.